Ultrasonic sensors are used in motor vehicles for parking assistance systems, for example, which during parking measure the remaining distance from obstacles bordering a parking space. Such parking assistance systems typically include one or multiple ultrasonic sensors and a control unit, the ultrasonic sensors containing an ultrasonic transducer which is used both for transmitting and receiving ultrasonic signals. In particular, a key functional requirement for the ultrasonic sensors is a so-called proximity measuring capability at a distance range of less than 30 cm.
German patent document DE 3 441 684 A1 discusses an electroacoustic transducer having a cap- or pot-shaped housing whose base surface is designed as a diaphragm. A piezoelectric ceramic body as a transducer element rests on the diaphragm, and is covered by a damping foam layer on the side opposite from the diaphragm. In order to generate sound radiation having a particularly small aperture angle in a vertical plane, but sound radiation having a relatively large aperture angle in a horizontal plane, the contour of the diaphragm and of the side wall surrounding same has an approximately elliptical, i.e., rotationally asymmetrical, design in a plane perpendicular to the direction of sound radiation. In such designs it is disadvantageous that, in particular in pulsed operation, not only the diaphragm but also the side wall is excited to oscillations of secondary modes.
German patent document DE 197 27 877 A1 discusses an ultrasonic transducer having a pot-like housing which has an annular wall, and a base surface as a vibrating diaphragm. A stabilizing ring is provided on the exterior of the wall to impart greater rigidity to the housing and limit the ultrasonic vibration essentially on the pot base as a vibrating diaphragm. However, in particular for pulsed transmission excitation of the ultrasonic transducer, this design results in excitation of secondary modes, which typically correspond to tilting and crumpling motions of the diaphragm pot wall. These secondary modes impair the proximity measuring capability of the ultrasonic sensor, since they prolong the attenuation time after transmission excitation, and beat patterns overlap with small reflected ultrasonic signals from nearby objects.
It is therefore desirable to further reduce the excitation of secondary modes in the transmission mode of ultrasonic sensors, thus improving the proximity measuring capability.